Handlebars.js

Handlebars.js is an extension to the Mustache templating
language created by Chris Wanstrath.
Handlebars.js and Mustache are both logicless templating languages that
keep the view and the code separated like we all know they should be.

Installing

Alternatively, if you prefer having the latest version of handlebars from
the 'master' branch, passing builds of the 'master' branch are automatically
published to S3. You may download the latest passing master build by grabbing
a handlebars-latest.js file from the builds page. When the
build is published, it is also available as a handlebars-gitSHA.js file on
the builds page if you need a version to refer to others.
handlebars-runtime.js builds are also available.

Note: The S3 builds page is provided as a convenience for the community,
but you should not use it for hosting Handlebars in production.

Usage

In general, the syntax of Handlebars.js templates is a superset
of Mustache templates. For basic syntax, check out the Mustache
manpage.

Once you have a template, use the Handlebars.compile method to compile
the template into a function. The generated function takes a context
argument, which will be used to render the template.

Helpers take precedence over fields defined on the context. To access a field
that is masked by a helper, a path reference may be used. In the example above
a field named link_to on the context object would be referenced using:

{{./link_to}}

Escaping

By default, the {{expression}} syntax will escape its contents. This
helps to protect you against accidental XSS problems caused by malicious
data passed from the server as JSON.

To explicitly not escape the contents, use the triple-mustache
({{{}}}). You have seen this used in the above example.

Differences Between Handlebars.js and Mustache

Handlebars.js adds a couple of additional features to make writing
templates easier and also changes a tiny detail of how partials work.

Paths

Handlebars.js supports an extended expression syntax that we call paths.
Paths are made up of typical expressions and . characters. Expressions
allow you to not only display data from the current context, but to
display data from contexts that are descendants and ancestors of the
current context.

To display data from descendant contexts, use the . character. So, for
example, if your data were structured like:

Whenever the block helper is called it is given one or more parameters,
any arguments that are passed in the helper in the call and an options
object containing the fn function which executes the block's child.
The block's current context may be accessed through this.

Block helpers have the same syntax as mustache sections but should not be
confused with one another. Sections are akin to an implicit each or
with statement depending on the input data and helpers are explicit
pieces of code that are free to implement whatever behavior they like.
The mustache spec
defines the exact behavior of sections. In the case of name conflicts,
helpers are given priority.

Partials

You can register additional templates as partials, which will be used by
Handlebars when it encounters a partial ({{> partialName}}). Partials
can either be String templates or compiled template functions. Here's an
example:

Comments

You can also use real html comments if you want them to end up in the output.

<div>{{! This comment will not end up in the output }}<!-- This comment will show up in the output --></div>

Compatibility

There are a few Mustache behaviors that Handlebars does not implement.

Handlebars deviates from Mustache slightly in that it does not perform recursive lookup by default. The compile time compat flag must be set to enable this functionality. Users should note that there is a performance cost for enabling this flag. The exact cost varies by template, but it's recommended that performance sensitive operations should avoid this mode and instead opt for explicit path references.

The optional Mustache-style lambdas are not supported. Instead Handlebars provides it's own lambda resolution that follows the behaviors of helpers.

Alternative delimeters are not supported.

Precompiling Templates

Handlebars allows templates to be precompiled and included as javascript
code rather than the handlebars template allowing for faster startup time.

Installation

The precompiler script may be installed via npm using the npm install -g handlebars
command.

If using the precompiler's normal mode, the resulting templates will be
stored to the Handlebars.templates object using the relative template
name sans the extension. These templates may be executed in the same
manner as templates.

If using the simple mode the precompiler will generate a single
javascript method. To execute this method it must be passed to
the Handlebars.template method and the resulting object may be used as normal.

Optimizations

Rather than using the full handlebars.js library, implementations that
do not need to compile templates at runtime may include handlebars.runtime.js
whose min+gzip size is approximately 1k.

If a helper is known to exist in the target environment they may be defined
using the --known name argument may be used to optimize accesses to these
helpers for size and speed.

When all helpers are known in advance the --knownOnly argument may be used
to optimize all block helper references.

Implementations that do not use @data variables can improve performance of
iteration centric templates by specifying {data: false} in the compiler options.

Supported Environments

Handlebars has been designed to work in any ECMAScript 3 environment. This includes

Node.js

Chrome

Firefox

Safari 5+

Opera 11+

IE 6+

Older versions and other runtimes are likely to work but have not been formally
tested. The compiler requires JSON.stringify to be implemented natively or via a polyfill. If using the precompiler this is not necessary.

Performance

In a rough performance test, precompiled Handlebars.js templates (in
the original version of Handlebars.js) rendered in about half the
time of Mustache templates. It would be a shame if it were any other
way, since they were precompiled, but the difference in architecture
does have some big performance advantages. Justin Marney, a.k.a.
gotascii, confirmed that with an
independent test. The
rewritten Handlebars (current version) is faster than the old version,
with many performance tests being 5 to 7 times faster than the Mustache equivalent.